1. Cross-reference to Related Applications
The subject matter of this application is related to that disclosed in copending applications application Ser. No. 08/633,712, now U.S. Pat. No. 5,756,990, filed on Apr. 17, 1996.
2. Field of the Invention
The present invention generally relates to a multi-hyperstereo method and system for the mitigation of image distortion from optical turbulence.
3. Description of the Prior Art
In the areas of horizontal, passive surveillance and target acquisition/identification, optical turbulence distortions can severely affect visible imagery and can significantly affect thermal imagery, especially when levels of identification are sought using modern improved resolution systems. If the current trend toward higher resolution for longer range detection continues, the impact of optical turbulence will increase as well. The adaptive optics approach used in astronomy does not have a counterpart for horizontal paths (that is, horizontal surveillance and target acquisition/identification) because there are no stars or guide stars to be used to drive a corrective mirror. The use of frame subtraction techniques does not work because of the random distortions of scene features occurring in individual images, as well as the long time intervals for obtaining average image blur.
One approach that has been used for the mitigation of aerosol-induced image blur involves a long-term (tens of seconds) time-average measurement of the aerosol modulation transfer function (MTF) that can be applied in near real-time to subsequent images because of the uniform nature of the scattering blur. See D. Sadot, et al., "Restoration of Thermal Images Distorted by the Atmosphere, Based on Measured and Theoretical Atmospheric Modulation Transfer Function," OPT.ENG. 33(1), pp. 44-53 (January 1994). However, the random nature of the optical turbulence distortions does not lend itself to the application of long-term, time-averaged MTF corrections of individual frames.
Experimental results showed that the use of hyperstereo imaging produced appreciable reduction of the optical turbulence distortions on objects viewed at 1-Km range with 10X visible stereo cameras with a 10-m platform separation. If a linear (or possibly area) array of cameras were used to view distant terrain, statistical comparison and integration of the multi-hyperstereo imagery could be used to mitigate the effects of optical turbulence in near real time. For example, if 1-m spacing between the individual cameras were used, the imagery would have comparable optical distortion statistics along the different camera lines-of-sight; but they would be uncorrelated.
The level of effort required to fully field the hardware and software necessary for such a technique is substantial due to the complexity of replicating, with appropriate algorithms, the manner in which human processing of live stereo video reduces the effects of optical turbulence distortions. The means of statistically averaging and correlating multiple line-of-sight imagery is even more complicated. However, the problem does not appear to be totally intractable because stereo vision is already being used in robotics for depth perception, although the ranges that are currently being used are only tens of meters, rather than hundreds of meters. See Takeo Kanade, "Development of a Video-Rate Stereo Machine," Proceedings of 94 ARPA Image Understanding Workshop, Nov. 14-16, 1994, Monterey, Calif.
Accordingly, it is clear that the areas of both surveillance and target acquisition/identification would be significantly benefitted by the reduction of the effects of optical turbulence distortion in terms of increased range and better target identification. This benefit is especially true for future aided target recognition systems. A passive technique would make the user of such surveillance and target acquisition/identification systems less detectable, as compared to the "active system" approaches.